1. Field of the Invention
The present invention relates to a solid state image pickup device using a charge coupled device (CCD) and, more particularly, to a solid state image pickup device in which a horizontal charge coupled device (HCCD) region is divided into multi-channels so as to improve efficiency of charge-transferring and to widen its dynamic range.
2. Discussion of the Related Art
Utilizing photoelectric conversion devices and CCDs, a general solid state image pickup device pictures a subject and outputs it into an electric signal. A charge coupled device (CCD) transfers signal charge generated in the photoelectric conversion device such as a photo diode, in a specific direction by using potential change in a substrate.
A general solid state image pickup device includes a plurality of photoelectric conversion regions such as photo diodes (PD), a vertical charge coupled device (VCCD) region formed between the plurality of photoelectric conversion regions, and transferring charge generated in the photoelectric conversion regions in a vertical direction; a horizontal charge coupled device (HCCD) region transferring the charge that is transferred from the VCCD region in the vertical direction, in a horizontal direction, and a floating diffusion region sensing and amplifying the charge transferred in the horizontal direction and outputting it to a periphery circuit.
A conventional solid state image pickup device will be explained with reference to the accompanying drawings.
FIG. 1a shows the interface of the a VCCD region and a HCCD region of a general solid state image pickup device.
A general solid state image pickup device includes a plurality of photoelectric conversion regions 1 for converting an image signal of light into an electrical signal; VCCD regions 2 formed between the photoelectric conversion regions 1 transferring in a vertical direction the image charge generated in the photoelectric conversion regions 1; first and second poly gates 3a and 3b repeatedly formed on the VCCD regions 2; and HCCD regions 4 transferring in a horizontal direction the image charge transferred from the VCCD regions 2.
At an interface of the VCCD and HCCD regions 2 and 4, the first and second poly gates 3a and 3b, which clocks of B1, B2, B3, and B4 are applied alternately to, are repeatedly formed. On the HCCD regions 4, the first and second poly gates 3a and 3b, which clocks of C1, C2, C3, and C4 are applied alternately to, are repeatedly formed. In an end stage of the HCCD regions 4, there is located a sensing amplifier 5 which senses and amplifies the image charge transferred and finally outputs it to a periphery circuit. That is, the image charge that had been generated in the photoelectric conversion regions 1 and then was transferred through the VCCD and HCCD regions 2 and 4, is outputted to the periphery circuit by means of the sensing amplifier 5. This image signal outputted to the periphery circuit goes through various signal-processing steps. The level of the image signal that has gone through such steps depends on charge-generating and accumulating abilities of the photoelectric conversion regions 1, as well as on charge-transferring abilities of the VCCD and HCCD regions 2 and 4 and charge-sensing abilities of the amplifier 5. However, the level of the image signal is not outputted as it is. Having get through the diverse signal-processing steps, it is outputted in limits of levels of an image display device of a television or the like. In levels of image signal of a television, the difference between a peak value of a synchronizing signal and a value of a luminance signal is 1.0 Vp-p. At this time, a luminance signal is limited within 714 mV, and a signal of more than 714 mV is clipped. In other words, even though signal charge generated in the photoelectric conversion regions 1 is more than 714 mV, it goes through the signal-processing steps so that it is clipped to 714 mV, thereby having a signal level of as high as 714 mV. Therefore, it looks an identical image with that of a signal level of as high as 714 mV.
In the image signal-processing steps, gamma correction reduces signal levels of more than 700 mV, thus enabling signal levels of up to 1300 mV to be displayed.
FIG. 1b depicts two conventional transfer functions. The first conventional transfer function is depicted by the dashed line 10 and represents the relationship between the photodiodes and the CCD as a whole, i.e., the photodiode voltage as the input and the CCD voltage as the output. The straight dashed line 10 indicates that the CCD transfers the photodiode voltage without attenuation.
The second conventional transfer function of FIG. 1b is depicted by the solid line 12. It represents the relationship between the CCD voltage and the voltage after the conventional image processing. Up to 714 mV, the image processing preserves the CCD voltage without attenuation. Above an input level of 714 mV, however, the image processing maps the CCD output voltage level as 714 mV, i.e., it forces the CCD voltage level to be no more than 714 mV. This results in a loss of detail concerning bright objects. On a monitor, this appears to the viewer as washout of a bright object.
A conventional solid state image pickup device, in which a dynamic range of image signals is adjusted during signal-processing in a periphery circuit, so as to output them into a screen display device of a television, has the following problem. Although a signal charge generated in a photoelectric conversion region is more than 714 mV, it is clipped to 714 mV by going through signal-processing steps, thereby looking like an identical image with a signal of 714 mV. There is increased only a screen seize rate of an image in a saturate state.